CHAPTER III 



GEOLOGICAL FORMATION 



VIEWED across the intervening channel f rom Colonsay, the land- 

 scape of Mull presents to us certain unfamiliar features which 

 find no counterpart in Colonsay or in any of the neighbouring 

 islands within view. The terraced outline of the majestic 

 Ben Mor is rounded and full, but, even where dissected into 

 summits and slopes, this Tertiary volcanic mass differs funda- 

 mentally in appearance from the hills of the southern islands, 

 which are composed of very ancient schistose rocks. 



In these two islands, Colonsay and Mull, we have types 

 sufficiently illustrative of the two main formations schistose 

 and gneissose on the one hand, and basaltic on the other 

 into which the Western Isles of Scotland may be grouped. 

 Colonsay, Gigha, Islay, Jura, and neighbouring islets are, as 

 might be expected from the trend of the great Caledonian 

 rent, closely associated in structure with the mainland of 

 Argyll. It is not certain, however, that the rocks of 

 Colonsay are actually represented among the schists of the 

 Argyllshire mainland, and it is interesting to note that Dr 

 Peach places them in the great Torridonian system, named 

 after Loch Torridon in Ross-shire. The rocks of Coll, Tiree, 

 lona, and the Outer Hebrides are more like the north of 

 Scotland gneisses. Skye, Canna, Eigg, Mull, and some 

 smaller islets comprise those of basaltic structure. 



While broadly placing them in a few groups, minor 

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